ProStar Sleeper Suspension Modification...

[Chamberpains]

Good to see you figured it out. They have an internal shaft that is often plastic or metal that either shears off or breaks its plastic weld to the barrel that allows air to the bags. Not all and actually very few fail with an audible air leak. Many, including your main suspension valves, will simply wear out over time and get stuck in a lower position.

Now that you know this. You may want to check your main suspension leveling valve for the same type of problem. Replace it if it's just as old as your cab valve.